The epic amount of rain that led to deadly, catastrophic flooding across large parts of South Carolina and North Carolina is an example of exactly the type of supercharged storm system climate scientists have been warning about for years as a likely consequence of global warming.

It's not that heavy downpours and floods didn't occur before manmade global warming became evident (for the record, they did). The issue now is that these events are even more severe than they otherwise would have been. And they are becoming more frequent in many areas.

Though there are significant limits on what climate scientists can say at this point about an event like the South Carolina floods, it's well-established that global warming has already led to a measurable increase in global atmospheric water vapor levels, and this moisture can be wrung out as heavier bursts of rain or snow.

It is also well-established in scientific literature that precipitation is increasingly falling in short, intense bursts rather than long-lasting, generally lighter events.

Percent changes in the annual amount of precipitation falling in very heavy events, defined as the heaviest 1% of all daily events from 1901 to 2012 for each region.

Image: National Climate Assessment

The risk for extreme precipitation events is increasing in many parts of the world.

One study, for example, showed that a 1-in-100 year winter-rainfall event in parts of the United Kingdom is already occurring more frequently, becoming a 1-in-80 year event.

This means that an event with a 1% chance of occurring each year now has a 1.25% risk of occurring in any particular winter, which translates to a 25% increase in risk, according to Oxford University scientists.

Water vapor and the Carolina firehose

While it's too soon to say precisely how global warming may have affected the rare confluence of events that conspired to dump at least 26.88 inches of rain in less than four days on South Carolina, changes in extreme precipitation events are one of global warming's most well-documented climate change impacts.

First, consider how significant this rainfall event has been. Charleston and Columbia saw so much rain in four days that they both blew past their previous records for the all-time wettest October, along with setting numerous other milestones.

Across South Carolina, nearly 400 roads and 158 bridges were closed on Monday due to the flooding that killed nine. More than two dozen evacuation shelters are open, and 30,000 residents are still without power.

The record-shattering rains in Charleston and Columbia, as well as other parts of North and South Carolina, were rarer than a 1-in-1,000 year event, based on recurrence intervals from the National Oceanic and Atmospheric Administration (NOAA). This means that they'd have just a 0.1% chance or less of occurring in a given year.

JUST IN: Charleston Int'l Airport had 17.70" of #rain the past 5 days, breaking the all-time 5-day record for the state of South Carolina.

As many climate scientists will say, though, we seem to be seeing more and more 500-year to 1,000-year events lately, to the point where the definition of such events and their return intervals may need to change.

How did this happen?

There were four main meteorological players in this extreme weather event, each of which raised the odds for a record rainfall event in the Carolinas and caused weather forecasters to sound the alarm for the Palmetto State as early as midweek last week.

These atmospheric players include an upper-level low-pressure area across the Southeast; a stationary front with an area of low pressure riding along it; Hurricane Joaquin, which passed off the East Coast and moved close to Bermuda; as well as a strong area of high pressure parked across southern Quebec.

These weather systems led to multiple moisture feeds directed at the Carolinas in firehose-like fashion for an unusually long time. Rainfall rates of 1-to-4 inches per hour or higher were recorded in the Charleston area and other locations for multiple hours at a time.

The creeping progress of the weather pattern, with the blocking high across Quebec playing a role in this, helped keep this firehose in place for days — from Thursday through Monday. Some studies have shown that stuck weather patterns have become more common in recent years, potentially tied to rapid Arctic warming, though this link is not accepted by all of the mainstream climate science community.

Think of some of these weather systems as pipelines directly connected to major moisture sources: the unusually mild waters of the tropical Pacific Ocean; milder-than-average waters of the western Atlantic; and a water-laden Hurricane Joaquin.

Climate change increases risk of extreme rainfall events

Across the U.S. as a whole, the frequency of 2-inch or greater rainstorms has increased in recent decades, and extreme precipitation events have been on the rise across the Northern Hemisphere as well.

A Climate Central analysis released in May found that 40 of the lower 48 states have seen an overall increase in heavy downpours (the days where total precipitation exceeded the top 1% of all rain and snow days) since 1950.

However, South Carolina was one of the few states that did not see an increase, based on that analysis. In fact, it saw a slight decrease in the frequency of heavy precipitation events. Neighboring states, including Georgia and North Carolina, did see increases, though.

Sea surface temperature departures from average in degrees C on October 4, 2015, with the area the heavy rain formed over circled in blue.

Image: WeatherBell Analytics

In general, warm air holds more moisture and evaporates more moisture from the soils and the ocean, which is also milder than it used to be. Ocean temperatures off the Southeast are above average for this time of year, which may have added more water vapor to this event, thereby giving the storm more energy to work with.

Specific so-called "climate attribution" studies examining the role that global warming may have played in this event may take several weeks to months for scientists to complete.

Michael Mann, who directs Earth Systems Science Center at Penn State University, told Mashable in an email that this storm is "yet another example, like Sandy, or Irene, of weather on 'steroids,' another case where climate change worsened the effects of an already extreme meteorological event."

"In this case, we’re seeing once-in-a-thousand year flooding along the South Carolina coastline as a consequence of the extreme supply of moisture streaming in from hurricane Joaquin," he said.

Image: Mic Smith/Associated Press

Joaquin intensified over record warm sea surface temperatures in the tropical Atlantic, which both allowed it to intensify rapidly despite adverse wind shear, and which provided it with unusually high levels of moisture — moisture which is now being turned into record rainfall.

Kevin Trenberth, a scientist at the National Center for Atmospheric Research (NCAR) in Colorado, is a prominent climate scientist who has been outspoken about tying global warming to the characteristics of extreme weather events. He has pointed out, in scientific papers and public appearances, that because global warming has added heat and moisture to the atmosphere, every weather event that occurs now is different than it would have been had it occurred several decades ago.

Image: Chuck Burton/Associated Press

For example, this storm occurred during a year that has at least a 97% chance of being Earth's warmest on record, and at a time when the planet's oceans are the warmest they've been since such records began in the late 19th century.

The tricky part for scientists lies in figuring out exactly how those changes manifest themselves, and how significant they are relative to other factors, such as natural climate variability.

"Global warming means more ocean heat and abundant surface water vapor waiting to be gathered by a storm," Trenberth wrote in an email to Mashable. A slow-moving storm such as this one, he said, "dumps it all in one place."

"This is increasingly occurring in different places and times and is to be expected from climate change," Trenberth said.

He also cited the ongoing strong El Niño event in the tropical Pacific, which has helped drive global ocean temperatures to their highest temperature on record, as a possible factor behind this storm system.

"El Niño means all action is in [the] Pacific," which suppresses tropical storm and hurricane activity in the Atlantic. "This means build up of heat that waits for an opportunity to escape in some sense."

Myles Allen, who leads the climate dynamics group at the University of Oxford and has participated in several climate attribution studies, was more cautious about linking the South Carolina extreme rainfall and flooding to global warming.

"All other things being equal, increasing atmospheric moisture resulting from large-scale warming would allow more such events to occur," he wrote in an email to Mashable.

"But other impacts of climate change, such as changing circulation patterns, could work in the opposite direction: so it’s an interesting question whether human influence on climate made this event more or less likely to occur, but not one on which we should jump to conclusions without doing the necessary analysis," he said.

Sea level rise makes matters worse

For coastal residents of the Carolinas, particularly those in Charleston, there was another climate change-related factor that helped accentuate the flooding: sea level rise.

Onshore winds associated with the squeeze play between a surface low-pressure area in the Southeast and the high-pressure area over the northeast helped drive high tides in Charleston to levels not seen since Hurricane Hugo made landfall there in 1989.

This exacerbated flooding caused by heavy rain, since the tide level prevented waters and sewage from draining out of the city at times.

According to Ben Strauss, a climate scientist at Climate Central who focuses on sea level rise, this flood event was a preview of what's to come for cities like Charleston as they face the prospect of more coastal flood events.

"This provides a mostly unrelated preview of what high tide could look like in Charleston after [the] sea level rise we could see later this century," Strauss wrote in an email.

Water levels for Charleston, South Carolina, showing the peak tide on October 4, 2015. Red marks water level above the predicted value.

Image: NOAA

Climate Central's sea level rise report for South Carolina found that more than 800 square miles of land lie less than 4 feet above the high-tide line in South Carolina, including $24 billion in property value.

Mashable
is a global, multi-platform media and entertainment company. Powered by its own proprietary technology, Mashable is the go-to source for tech, digital culture and entertainment content for its dedicated and influential audience around the globe.